1. Field of the Invention
The present invention relates, in general, to the desulfurization of flue gas and, in particular, to a new and useful method for converting a double-loop flue gas desulfurization system to a single-loop flue gas desulfurization system.
2. Description of the Related Art
It is well-known that there are numerous flue gas desulfurization (FGD) systems available and that the majority of these systems fall under two categories. The first category is the single loop system. In single loop systems, the recirculation tank and the scrubbing zone are combined in one structure. The liquor sprayed in the scrubbing zone captures SO.sub.2 and looses its alkalinity. The pH of the liquor leaving the scrubbing zone falls to about pH 4.5. The low pH scrubbing liquor then falls into the recirculation tank where it mixes with the large volume of alkaline slurry in the tank and the pH of the recirculated liquor is raised to the process set point. Fresh slurry is added to control the pH of the liquor in the tank to preset relatively acidic pH varying from 5.8 to 6.2.
The recirculation tank volume is usually set to allow a certain solids residence time in the tank. The longer the residence time of the solids in the tank, the higher is the reagent utilization. Improved limestone utilization translates into more economic scrubbing.
The gas inlet of such systems is usually at an elevation above the liquid level in the tank which may be 25 to 30 feet above ground level in order to accommodate the solid residence time requirements.
The second category is the double-loop system. In this system, there are two loops which are virtually separate from each other. The scrubbing loop contains the fresh alkaline slurry. The slurry is then sprayed over several layers of packing to enhance the SO.sub.2 removal capability of the system. The contact between the liquor on the packing and the flue gas causes the pH of the slurry leaving the packing to drop. This is similar to the single-loop system.
Then, the reacted slurry, which is fairly low in pH, is collected in a bowl and returned to an external tank. The same parameters which govern the tank volume and size in the single-loop system also apply to the dual-loop system. Therefore, the size of the external tank is approximately the same as the size of a recirculation tank in a single-loop system.
Fresh slurry is added to the recirculation tank to maintain the process set pH and the adjusted pH slurry is recirculated from the recirculation tank to the spray zone over the packing. Slurry from the recirculation tank overflows into the bottom of the scrubber, under the bowl, and is recirculated through a second loop to a set of headers, also located under the bowl. The function of these headers is to humidify the flue gas entering the scrubber so that wet/dry interface deposits do not form and assist in the scrubbing process. Exhausted liquor is usually drawn from the bottom of the scrubber based on the preset level.
The lower loop usually runs at a lower pH than the upper loop enhancing limestone utilization and reducing operating costs.
The level of liquid in the integral scrubber tank of a dual loop system is about 8 to 10 feet which places the gas inlet about 15 to 20 feet below the gas inlet of a single-loop system. Accordingly, this presents a problem to owners who want to upgrade a dual-loop system to a single-loop system.
Dual-loop systems are prone to many problems because of the separation of the loops. For example, the operator of a naturally oxidized dual-loop system does not have the flexibility to benefit from burning low sulfur coal. Low sulfur coals tend to have higher natural oxidation in a naturally oxidized system than high sulfur coal. Oxidation rates between 15% and 90% generally result in scaling and plugging of the packing, which reduces the reliability of the system and SO.sub.2 removal capability as the plugging progress. In addition, the system resistance is variable and dependent on the packing cleanliness and condition.
Due to the high SO.sub.2 removal of the lower loop, when organic acids are used, a dual loop system tends to plug the packing with scale build up. Single loop systems using organic acid buffers are immune to this problem.
Due to the use of a shallow integral tank, dual-loop systems are difficult and expensive to convert to a single-loop system if the same external recirculation tank residence time is required. Owners of dual-loop system who have problems with packing plugging have a hard time cleaning the packing section, which is normally 2 or 3 feet deep, without causing extensive damage to the packing. They also have a hard time converting the dual-loop system to use in-situ forced oxidation in order to convert the calcium sulfite formed into Gypsum.